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AAPG Bulletin

Abstract


Volume: 62 (1978)

Issue: 6. (June)

First Page: 1029

Last Page: 1048

Title: Morphology and Structure of Upper Continental Margin Off Southern Brazil

Author(s): John D. Milliman (2)

Abstract:

The shelf and upper slope off southern Brazil are underlain by two broad sedimentary basins, the Santos in the north and the Pelotas in the south. The landward edge of maximum sediment thickness in the basins is delineated by the Rio Grande-Santos fault, which apparently formed during (or subsequent to) early rifting of the South Atlantic. Both basins also appear to be divided into several north-south sections by the intersection of east-west fracture zones.

Although depth to basement in both basins exceeds 7 km, the lower basin fill consists of Cretaceous lava (Serra Geral). In addition, the Santos basin contains Aptian salt overlying the Serra Geral. Thus, total post-Aptian terrigenous accumulation in the Santos basin is generally less than 4 km, mostly derived from erosion of the adjacent Serra do Mar; at least some of the Pelotas sediments were transported northward from the Rio de La Plata. Unconsolidated sediment, assumed to be post-Miocene in age, is markedly thinner in the Santos basin than the Pelotas, possibly because of continued retreat of the Serra do Mar.

Shallow seismic profiling shows that the upper strata in the Santos basin accumulated as several cycles of foreset beds, the greatest thickness being in the northern part. Comparison with available drilling and eustatic data suggests that these strata were deposited in the middle Tertiary, prograding over an Eocene unconformity. In contrast, sediments in the Pelotas basin show no sign of foreset deposition but, rather, suggest accumulation in near-shore environments (notably lagoons and estuaries) similar to the coastal environments along the present shoreline. Uplift in the Pelotas basin may have occurred as recently as the late Tertiary-early Quarternary (as inferred from seismic profiles over the Patos arch), followed by extensive erosion and subsequent sedimentation.

Text:

INTRODUCTION

In 1972 the Woods Hole Oceanographic Institution initiated an agreement with a consortium of Brazilian companies, agencies, and universities, termed REMAC (FOOTNOTE 3), for the cooperative investigation of the geology and oceanography of the upper Brazilian continental margin. The program, which centered around eight oceanographic cruises along the entire margin, consisted of three major programs. Much of the effort was used in collecting and analyzing surficial-sediment samples from the shelf and upper slope to gain a picture of sources, distributions, and economic potential of the sediments. Results of these studies have been summarized in a monograph edited by Milliman and Summerhayes (1975).

The second part of the program consisted of studying the suspended particulates in the surface waters off Brazil; results were reported in a series of articles in the Journal of Sedimentary Petrology and summarized (along with other Atlantic Ocean data) by Emery and Milliman (1978).

The third part of the Brazilian program dealt with the morphology and shallow structure of the upper margin. A 3.5-kHz echo sounder was used to record bathymetry and structure of the upper 10 to 50 m of sediment. A towed magnetometer recorded the magnetic field of the shelf and upper slope; the results of this investigation were reported by Fainstein et al (1975). The shallow structure was studied using a continuous seismic profiler (8-10-Kj 10-electrode sparker) with summed twin hydrophone arrays. The sparker was fired every 12 sec, seismic profiles being recorded on two stop-start x-y recorders, one with a 1-sec sweep and the other with a 2-sec sweep. Normal ship speed during profiling was 8 knots; navigation was by satellite. This paper discusses the morphology,

FOOTNOTE 3. Reconhecimento Global da Margem Continental Brasileira, composed of Petroleo Brasileiro, S.A., Companhia de Pesquisa de Recursos Minerais, Departamento Nacional de Produca Mineral, and Conselho Nacional de Pesquisas.

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structure, and evolution of the southern Brazil margin. Similar papers deal with the structure and morphology off northeastern Brazil (Summerhayes et al, 1976) and the Amazon (Milliman, in prep.).

MORPHOLOGY OF SOUTH BRAZILIAN MARGIN

The continental shelf off southern Brazil is widest (180 km) off Sao Paulo and Rio Grande do Sul, and narrowest (less than 80 km) off Rio de Janeiro and Santa Catarina states (Fig. 1). Declivities across the shelf average between 1 and 2 m/km, with the depth at the shelf break increasing from 120 m in the north, to 160 m in the south.

The most obvious surface feature on the shelf is a series of longitudinal depressions along the middle shelf of Rio Grande do Sul. In the far south these depressions closely resemble (in both configuration and location) the modern coastal lagoons of Rio Grande do Sul, and are filled with horizontally bedded sandy muds (Fig. 2a). Further north, off Porto Alegre, the mid-shelf mud lens thins both landward and seaward, quite dissimilar from the profiles in the south (compare Figs. 2a, b). Mud thickness (as detected by 3.5-kHz echo sounding) along this entire southern mud belt exceeds 18 m, as contrasted to subbottom penetration of 1 to 5 m in the inner and outer shelf sands (Figs. 2, 3).

Although there is little surface expression of relict river and stream channels crossing the shelf, 3.5-kHz records show filled channellike depressions (Fig. 2d, e), one of which can be traced across the shelf seaward of Lagoa dos Patos (Fig. 3). Another indication of lower stands of sea level is two "scarps," supposedly Flandrian transgressive shorelines, at 60 and 110 m; locally these shorelines have been buried by Holocene sediments (Kowsmann et al, 1976).(FOOTNOTE 4) The muds filling both the depressions in the south and covering the shorelines in the north were suggested to be lagoonal (de Rocha et al, 1975), but subsequent analysis showed that at least the surficial sediment is marine and was deposited during the last rise in sea level (Kowsmann et al, 1976).

The continental slope extends from the shelf edge to approximately 2,000 m off Rio de Janeiro and Sao Paulo, at which depth the Sao Paulo Plateau begins. Farther south, the slope-rise boundary deepens to more than 2,800 m off northern

Fig. 1. Bathymetric chart of southern Brazil based on soundings obtained during REMAC cruises in 1972-73, plus considerable data from General Bathymetric Chart of the Oceans (GEBCO) sheets. Intricate bathymetric configurations on much of shelf are defined on basis of detailed echo sounding by Petrobras. Depths are plotted in corrected meters.

FOOTNOTE 4. The depth and character of these scarps corresponds with the Fortune and Franklin shorelines off the northeastern United States (e.g., Emery and Uchupi, 1972), and also with similar shorelines off Argentina (Fray and Ewing, 1963).

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Uruguay. The slope varies greatly in declivity, from less than 10 m/km in some areas, to more than 40 m/km off Sao Paulo. Modern, unconsolidated sediment cover on the slope (as indicated by 3.5-kHz echo sounding) is patchy (Figs. 2, 3). In some places abrupt changes in declivity on the slope are defined by outcropping strata (Fig. 2g), but in others the entire slope is characterized by eroded tilted beds (Fig. 2h).

In contrast to other areas on the Brazilian continental margin, the shelf edge and slope off southern Brazil do not appear to be cut by many canyons or gullies, although several large gullies (termed the Rio Grande Valleys by Zembruscki et al, 1972) are present off southern Rio Grande do Sul. Apparently these canyons are Tertiary in age and presumably inactive, because their landward extensions appear to have been covered by subsequent Quaternary sedimentation as described later. Evidence of more recent cutting of surficial sediments, however, is present on the upper slope off Sao Paulo, where several gullies cut surficial strata (Fig. 2f).

TECTONIC SETTING

Butler (1970) and Miranda (1970) provided excellent syntheses of the marine and terrestrial geology known at that time. However, more recent observations, such as seismic refraction profiles by Lamont-Doherty Geological Observatory (Leyden et al, 1971, 1976; Francisconi and Kowsmann, 1976; Kowsmann et al, 1977a), magnetic anomaly patterns (Fainstein et al, 1975; de Rezende et al, 1977) and further land-based studies (e.g., Almeida, 1976) have added greatly to our knowledge of the area. The most recent comprehensive synthesis of data was by Kowsmann et al, 1977a), although unpublished information provided by colleagues at REMAC allows some new interpretations in the present discussion.

On the basis of terrestrial geology, southern Brazil can be divided into four major sections (Fig. 4):

1. Much of the coast north of Rio Grande do Sul is bordered by the Precambrian Atlantic folded system of metasedimentary sequences associated with basic and acidic magmatic intrusives and volcanic rocks (Delaney, 1966; Ferreira, 1972; Almeida et al, 1973; Campos et al, 1974). The eastern edge of this plateau ends in steep scarps, 800 to 2,200 m in relief, commonly referred to as the Serra do Mar (Almeida, 1976). The Precambrian folded system also is characterized by a number of northeast-southwest faults (some reactivated Precambrian faults) that formed during Early Cretaceous rifting (Braun, 1975; Almeida, 1976; see following). The faults are generally normal but some are transcurrent. Contemporaneous with the faulting was a major uplift of the Serra do Mar; erosion of this range and subsequent landward retreat provided the probable source of Tertiary sediments for the Santos basin (Butler, 1970; Fulfaro, 1975; Almeida, 1976).

2. Inland of the Precambrian belt lies the Parana basin, containing late Paleozoic through Cretaceous sedimentary rocks predominantly continental in origin.

3. One of the most prominent features in southern Brazil is the Serra Geral basaltic lava, which has filled much of the Parana basin, nearly reaching the Atlantic Ocean at the Torres syncline (Fig. 4); thicknesses range up to 1,500 m. Extensive dating of these basalts indicates a mean age

Fig. 2. Line drawings of 3.5-kHz echo soundings off southern Brazil. Profiles a-c and g, h are normal to shelf edge; others parallel shelf. All profiles except g have horizontal scale of 3 km; g has scale of 8 km.

a, Possible fossil lagoon off Lagoa dos Patos; b, mud lens prograding seaward off Porto Alegre, at north end of Lagoa dos Patos; c, possible buried terrace off Sao Paulo; d, buried stream channel seaward of southern entrance to Lagoa dos Patos--location of this channel is shown in Figure 3; e, possible buried stream channel off southern Parana State; f, profile along upper slope off Sao Paulo, showing gully erosion of younger beds (left); locally gullies erode into older (Pleistocene?) strata (right); g, changes in declivity on upper slope off Sao Paulo are defined locally by outcropping of strata; h, in other places, slope off Sao Paulo contains outcropping strata, with little or no sediment cover.

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Fig. 3. Depth penetration by 3.5-kHz echo sounder during REMAC cruises off southern Brazil shows middle shelf mud patches off Rio Grande do Sul and well-defined buried stream channel (heavy line) that crosses shelf seaward of southern entrance to Lagoa dos Patos.

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Fig. 4. Basement-structure map of southern Brazil showing general land geology and depth to basement (in kilometers) on continental margin. (Data from Ewing et al, 1969; Departamento Nacional da Produca Mineral, 1971; Leyden et al, 1971; Asmus and Ponte, 1973; Francisconi and Kowsmann, 1976; Leyden et al, 1976; Kowsmann et al, 1977a; H. A. Ojeda e Ojeda, unpub. ms.)

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of 120 m.y. (Amaral et al, 1966; Melfi, 1967; Pacca and Hiodo, 1976), coincident with the initiation of Early Cretaceous rifting in the South Atlantic (Almeida, 1976).

4. The southern Brazil coast is bordered by small sedimentary basins, post-Jurassic in age, comprising the landward edges of two major offshore basins, the Santos in the north, and the Pelotas in the south. Although much of the subsequent discussion deals with these offshore basins, the landward parts are better known in that they have been extensively mapped and drilled. On land the oldest sediments in the Pelotas basin are Miocene (as opposed to Cretaceous ages for basal sediments offshore; Kowsmann et al, 1977a), the entire sequence alternating between mixed marine and brackish facies (Closs, 1970). Sediment thickness of the shoreward part of the Pelotas basin, as delineated by a coastal well at Mostardes (Rio Grande do Sul State), is more than 1.5 km. Sediments in the landward ext nsion of the Santos basin generally are only hundreds of meters thick.

Generally the same features present on land also are present on the shelf. The northeast-southwest faults on land seem to parallel the major "fault" system that extends along the entire middle shelf of southern Brazil (Fig. 4). This system, known as the Rio Grande fault in the far south and the Santos fault in the north, may mark the contact between continental and oceanic crust (Kowsmann et al, 1977a). The fault system coincides with the trend of magnetic anomaly G on the middle shelf of northern Argentina and Uruguay, which Rabinowitz and LaBreque (1977) believe reflects the magnetic-edge effect at the contact between oceanic and continental crusts.

The Rio Grande-Santos fault system is well delineated by a sharp boundary in magnetic-anomaly patterns on the shelf. Nearshore anomalies are characterized by short wavelengths and high amplitudes (locally greater than 800 gammas), whereas anomalies on the middle and outer shelf, particularly in the area of the Santos and Pelotas basins, are broader and lower in amplitude (Fig. 5). Presumably the more subdued magnetic anomalies illustrate the greater sediment thicknesses on the outer part of the shelf. The pronounced

Fig. 5. Magnetic anomalies on southern Brazilian upper continental margin based on measurements taken during Woods Hole Oceanographic Institution-REMAC cruises in 1972-73, after Fainstein et al (1975). De Rezende et al (1977) suggested that dramatic shifts in magnetic patterns off Rio Grande do Sul may reflect landward contact of oceanic fracture zones, depicted here.

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anomalies on the inner shelf may represent basaltic dikes from the Serra Geral lava (Leyden, 1976; Leyden et al, 1976). Interestingly, the high-amplitude, short-wavelength anomalies intersect the coast near Porto Alegre, indicating a structural lineation not detected in previous studies (Fig. 5). On the basis of deep seismic data from Petrobras, de Rezende et al (1977) suggested that some of the magnetic anomalies on the upper margin result from the intersection of the margin by oceanic fracture zones. Possibly the high-amplitude anomaly off Porto Alegre indicates intersection of the Rio Grande do Sul fracture zone (Fig. 5).

Maximum sediment thicknesses in both the Santos and Pelotas basins are seaward of the Rio Grande-Santos fault. Depth to basement (assumed on the shelf to be represented by seismic velocities of 5.7 to 6.0 km/sec) and in oceanic areas by velocities of 5.3 to 5.7 km/sec) is greater than 7 km in the Santos basin and more than 9 km in the Pelotas basin immediately seaward of the shelf edge (Fig. 4).(FOOTNOTE 5)

A section across the Santos basin allows a more detailed comparison of seismic velocities and exploratory well data, and thus a better insight to the nature of both offshore basins (Fig. 6). Of particular interest is the apparent correlation of the 4.17 to 4.32-km/sec layer in the Santos basin with salt of Wealdian and Aptian age (Leyden et al, 1976). Possibly the lower part of this layer contains basaltic strata similar to those underlying the salt in several of the exploratory holes (Fig. 6). If correlative with the Serra Geral formation, these underlying basalts should be Early Cretaceous in age, a supposition supported by a radiometric age of 121 m.y. for a lava sampled in one offshore well (Leyden et al, 1976). Some prerift sediments may underlie or intermingle with this basaltic layer (Leyden et al, 1971; Asmus, 1975).

Sediment thickness above the Aptian salt shows considerable relief (Fig. 7), reaching greater than 3 to 4 km along the central part of the Santos basin. Deviations from horizontal bedding may indicate plastic flow of salt, a suggestion supported by the prominence of salt diapirs in the area, particularly on the Sao Paulo Plateau (Leyden and Nunes, 1972; Leyden et al, 1976).

Correlation with exploratory well logs indicates that the velocity discontinuity between the 2.35 to 2.49 and 3.11 to 3.47-km/sec layers may represent the contact between Cenozoic and Mesozoic strata, as noted on other continental margins (E. Uchupi, oral commun.). However, a more obvious acoustic discontinuity is the one between the 2.35 to 2.49-km/sec layer and the unconsolidated sediments (those with velocities equal to, or less than 2.0 km/sec). These latter sediments (apparently post-Miocene; Fig. 6), account for up to 0.5 km of sediment in the Santos basin, but more than 3.0 km in the Pelotas basin (Fig. 8).(FOOTNOTE 6) Presumably the greater sediment thickness in the far south is due to deposition of detritus from the Rio de la Plata (see following), whereas the sparse unconsol dated cover in the Santos basin may have resulted from retreat and decreased uplift of the Serra do Mar, and thus lessened sediment influx.

SHALLOW STRUCTURE

Santos Basin

Nature of the basin fill, particularly the upper 1 km of section, is shown best by the high-resolution seismic profiler records in Figures 9 to 13. In most cases we are looking at middle and late Tertiary sequences. Profiles 41 (Fig. 9), 15, and 19 (Fig. 15) show the shallow depth to basement both north (off Rio de Janeiro) and south (Santa Catarina) of the basin. In the former case, the strong reflection of acoustic basement, plus the correlation with seismic refraction (and sonobuoy) data, suggests that basement is the 5-km/sec layer, probably Precambrian metasedimentary rock. Although acoustic basement deepens both offshore and to the east (toward the southwestern edge of the Campos basin, which lies on the north) it also may be Precambrian basement (profile 46, Fig. 9).

North of approximately 25°S lat., the outer shelf is underlain by two or more sets of foreset beds, buried by at least 0.2 sec of horizontal strata. Butler (1970) suggested that this sequence of prograding beds accumulated by relatively rapid subsidence of the basin. As mentioned by Butler, thickness of the foreset sequence increases north of Sao Paulo, four or five cycles being detected in several profiles, with total thicknesses in excess of 0.5 sec (e.g., profiles 44 and 36, Fig. 10). Closer inspection of the profiles shows a minor unconformity between the foreset sequences (Fig. 12), as is explained later. On the south, generally only one or two cycles of foresets are present (compare Figs. 10, 12). The northern foresets tend to form the shape of the shelf-slope boundary, and the

FOOTNOTE 5. This latter observation conflicts with that of Butler (1970), who speculated that the Pelotas basin has relatively little offshore expression off southernmost Brazil and northern Uruguay.

FOOTNOTE 6. The thickness of 3 km is based on the results of a single sonobuoy, but 5 others show thicknesses in excess of 1.5 km, thus confirming the greater unconsolidated cover in the Pelotas basin.

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Fig. 6. Cross sections of Santos basin, B-B1, and Pelotas basin, A-A1, based on sonobouys (Pelotas basin) plus two-ship seismic refraction and exploratory well data (Santos basin). Data from Ewing et al (1969), Leyden et al (1971), Kowsmann et al (1977a), and Petrobras unpublished data. Locations of sections shown in Figure 4.

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Fig. 7. Depth to top of 4.17 to 4.32-km/sec layer in Santos basin. Top of this layer presumably is Aptian salt, and sediments above it are post-Aptian in age.

Fig. 8. Thickness of sediments with velocities less than, or equal to 2.00 km/sec in Santos and Pelotas basins. Presumably these unconsolidated sediments are post-middle Miocene in age. Data from same sources as in Figure 6.

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Fig. 9. Profiles off northern edge of Santos basin. Acoustic basement may be 5-km/sec layer.

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Fig. 10. Profiles across northern Santos basin showing prominence of prograding foreset beds.

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Fig. 11. At top is original record from 10-Kj sparker high-resolution seismic profiler, with interpretation of record below (this profile, 36, is also in preceding illustration in foreshortened form) showing marked angular unconformity (1), probably Eocene; less obvious unconformity (2), probably separating Oligocene and Miocene foreset beds; and growth faults (circled) that characterize many profiles in Santos basin outer shelf and upper slope. At bottom is line interpretation of profile 29, across upper slope off Sao Paulo State (see Fig. 13 insert for approximate location), showing only diapir and related faults seen on these cruises.

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Fig. 12. Profiles across central and southern sections of Santos basin.

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more southern beds tend to be buried by the prograding shelf. The difference in character between the northern and southern parts of the Santos basin also can be seen in CSP records parallel with the shelf break: in the north, foreset bedding trends northward toward the axis of greatest foreset development, whereas in the south the shelf is characterized by numerous eroded and filled channels (Fig. 13).

Correlation of these seismic profiles with seismic refraction and drill-hole data suggests that the foreset beds are middle Tertiary, quite possibly deposited during several worldwide regressions (Vail and Mitchum, in Gussow, 1976). The previously mentioned unconformity within the foreset sequence (Fig. 11) may reflect a eustatic hiatus in deposition in the Oligocene and late Miocene (E. Uchupi, oral commun.). Some of the topset strata may be continental, but presumably most are shallow-marine (Ojeda e Ojeda, 1973).

Profile 35 (Fig. 12) gives evidence that the upper slope surface results directly from the progradation of the foreset beds. The shaping of the shelf-slope interface during the late Miocene regression agrees with observations off the eastern United States, where the entire upper slope appears to lie on an Eocene erosion surface (J. Austin, in prep.).

Because much of the seismic profiling was confined to the shelf and upper slope, salt diapirs, which characterize the subsurface structure of the slope and adjacent Sao Paulo Plateau (Leyden, 1976), are present in only one profile (Fig. 11). As noted by Butler (1970), the diapir field is marked by extensive faulting and the surface of the slope has numerous examples of collapse features. Other faults noted in CSP records tend to be related more to sedimentary processes, either growth faults (e.g., Fig. 11), or to down-slope movement in areas of slumping (e.g., profiles 34, 35, Fig. 12).

Pelotas Basin

The shelf off Rio Grande do Sul State is characterized by its width, and in subsurface by a broad intrabasinal arch within the Pelotas basin. The name "Patos arch" is proposed, although quite probably it represents a seaward extension of the Sao Gabriel arch. On the upper slope, this arch is delineated by exposed truncated beds (profile 3, Fig. 14). Although incision by one of the Rio Grande valleys partly obscures the record, strata dip both north and south. On the outer shelf the arch is overlain by 0.15 sec of sediment. Seismic patterns of the overlying strata suggest deposition in shallow water (profile 7a, Fig. 14). There is no surface expression of the Rio Grande valleys, but the presence of a buried channel indicates that the valleys may have been buried. A profile farther insh re (profile 5, Fig. 14) shows no evidence of the arch; in all likelihood it is overlain by more than 0.4 sec of sediment and connects with the Sao Gabriel arch on land.

Although we know that the strata are deeper south of the Patos arch (profile 3, Fig. 14), the lack of profiles in the south prevents speculation about the nature of the southern part of the Pelotas basin. Presumably maximum thicknesses are present on the outer shelf and upper slope (Fig. 4). North of the arch strata also deepen, only to shoal against the Torres arch (Kowsmann et al, 1977a; Figs. 4, 7). No east-west fault was seen to form the northern edge of the basin (suggested by Delaney, 1966), although deeper seismic profiles (de Rezende et al, 1977) and magnetic data suggest

Fig. 13. Profiles along outer shelf of Sao Paulo and Parana, showing different depositional character in northern and southern parts of Santos basin.

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Fig. 14. Profiles across Patos arch.

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that such a series of deeper faults may be present, associated with the Rio Grande do Sul fracture zone (Fig. 6).

Shallow-water depositional structures noted in some profiles (e.g., profile 15, Fig. 15) do not appear to be so prominent as those further south. On the outer shelf, strata actually crop out in the vicinity of the Torres arch (profiles 17, 19, Fig. 15), suggesting that uplift and erosion continued after initial uplift of the arch (120 m.y. ago?). Alternatively, these eroded strata may indicate only the effects of lower stands of sea level, in which case the Torres arch would have been inactive for some time.

Several profiles normal to the shelf edge show evidence of features analogous to buried lagoons. This is seen best on the middle shelf (upper 0.2 sec) of profile 2 (Fig. 16). Other indications of shallow-water features, such as filled channels (profile 8, Fig. 16) and foreset bedding (profile 10, Fig. 16), are present in the upper strata of other transverse sections.

Profiles perpendicular to the shoreline also illustrate that the shelf-slope is characterized by upbuilding, in contrast to the progradation off the Santos basin. Only in the southernmost and northernmost profiles is there significant outbuilding (profiles 2, 18, Fig. 16). Although the slopes are largely constructional, several profiles (10, 18) show signs of erosion. Slumping, although locally important (profile 7b, Fig. 15), does not appear so widespread as off the Santos basin. However, sparse data (particularly on the lower slope) do not allow us to conclude that slumping is not widespread.

DEVELOPMENT OF SOUTHERN BRAZILIAN CONTINENTAL MARGIN

Understanding the structure and history of the southern Brazilian continental margin is relatively easy compared to many other Atlantic margins by virtue of the comparatively thin sediment cover and accessibility to excellent geophysical and exploratory well data about the area. The picture that has emerged may well form the basis for a better understanding of other Atlantic-type margins in which the sedimentary sequence is thicker or the data are fewer.

During initial rifting of South America from Africa, about 120 m.y. ago, uplifting of crystalline Precambrian rocks (Serra do Mar) and deposition of lava flows (Serra Geral) formed sources for subsequent sediment influx into the newly formed margin. With continental separation and initiation of restricted oceanic circulation, an extensive evaporitic(?) salt sequence was deposited for approximately 15 m.y.; normal oceanic circulation was prevented by the presence of the Rio Grande Rise and Walvis Ridge on the south, and Africa-South America on the north. Leyden (1976) postulated that the salt was deep-water in origin, but the possibility of shallow-water evaporation cannot be dismissed. Presumably, Aptian sedimentary strata south of the salt zone are normal-marine.

Salt formation ceased when the Rio Grande Rise and Walvis Ridge split, at the end of the Aptian (105 m.y. ago), and a shallow-marine clastic sequence, intermingled with calcareous strata, began to accumulate. Perhaps as a result of increased sediment burden, or perhaps (more likely) during initial rifting, normal faults continued to develop along zones of weakness, both on land and along the newly formed margin. The midshelf fault zone (the Rio Grande-Santos fault) may represent one such fault, or it may mark the transition from oceanic to continental crust. In any event, downfaulting on midshelf resulted in the formation of two sedimentary basins, the Pelotas in the south and the Santos in the north. Subsequent intersection of fracture zones against the margin resulted in a series of deeply seated east-west faults that cut across the Pelotas basin (de Rezende et al, 1977); similar faults may cut the Santos basin.

Although a plot of depth to basement looks impressive (Fig. 4), less than 3 to 4 km of terrigenous sediment (i.e., since the end of Aptian salt deposition) has accumulated in the Santos basin (Fig. 7). Presumably, total accumulation in the outer part of the Pelotas basin has been greater. Considering that present drainage in the Serra do Mar is mostly westward (into the Parana-Rio de la Plata River complex), perhaps it is surprising that so much sediment did accumulate.

The individual histories and developments of the Pelotas and Santos basins have been somewhat different. First, margin building has been primarily outward in the Santos basin and upward in the south. At least the upper part of the sediment in the Santos basin was deposited as a series of foreset beds (probably during late Tertiary transgressions and regressions), the obvious source of sediment being the Serra do Mar. Second, although sediment accumulation during the post-Miocene has been relatively light in the Santos basin (generally less than 0.5 km of sediment), it has accounted for a large amount of unconsolidated and uncompensated sediment load in the Pelotas (locally greater than 3 km; Fig. 8). Presumably, the difference in these rates reflects retreat of the Serra do Mar and th resulting decrease in erosion and sediment influx in the north. In contrast, the Rio de la Plata appears to

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Fig. 15. Profiles parallel with shelf break in Pelotas basin. Faults shown in profile 11 may not be real, but may reflect velocity changes just under filled channels.

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Fig. 16. Profiles across Pelotas basin.

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contribute significant material to the Pelotas basin in the south (see de Rocha et al, 1975; Milliman and Santana, 1976).

Tectonic activity in these basins has been relatively sparse. Several upwarpings are present in the Pelotas basin, the most notable being the Patos arch. No development of an interbasinal high was seen in the Santos basin, although perhaps this reflects the limited vertical penetration of our CSP rather than the lack of tectonic activity.

Slumping has been locally intensive, and several profiles show as much as 0.5 sec of slumped material on the middle and lower slope. Faulting is present in some profiles, the result of both diapir activity (in the north) and down-slope mass movement of sediment.

Quaternary sedimentation appears to have occurred primarily during lower stands of sea level, during transgression and/or regression. This is indicated both by the dominance of relict sediments on the shelf and also by several relict morphologies that reflect their shallow-water origin (such as lagoons and buried shorelines).

References:

Almeida, F. F. M. de, 1976, The system of continental rifts bordering the Santos basin, Brazil, in Continental margins of the Atlantic type: Acad. Brasileira Cienc. Anais, v. 48 supp., p. 15-26.

Almeida, F. F. M. de, et al, 1973, The Precambrian evolution of the South American cratonic margin south of the Amazon River, in A. E. M. Nairn and F. G. Stehli, eds., The ocean basins and margins, v. 1, The South Atlantic: New York, Plenum Press, p. 411-446.

Amaral, G., et al, 1966, Potassium-argon dates of basaltic rocks from southern Brazil: Geochim. et Cosmochim. Acta, v. 39, p. 159-189.

Asmus, H. E., 1975, Controle estrucural da deposicao Mesozoica nas bacias da margem continental Brasileira: Rev. Bras. Geocien., v. 5, p. 160-175.

Asmus, H. E., and F. C. Ponte, 1973, The Brazilian marginal basins, in A. E. M. Nairn and F. G. Stehli, eds., The ocean basins and margins, v. 1, The South Atlantic: New York, Plenum Press, p. 87-133.

Baccar, M. A., 1970, Evidencias geofisicas do pacote sedimentar no plateau de Sao Paulo: Cong. Brasileiro Geologia, no. 24, p. 201-210.

Braun, O. P. G., 1975, Structure of the coastal orogenic-belt of southeastern Brazil (abs.), in Continental margins of the Atlantic type--international symposium: Univ. Sao Paulo, Brazil.

Butler, L. W., 1970, Shallow structure of the continental margin, southern Brazil and Uruguay: Geol. Soc. America Bull., v. 81, p. 1079-1096.

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Acknowledgments:

(2) Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543.

The research reported in this paper was supported primarily through funds supplied by REMAC. Additional funding came from the Office of International Decade of Ocean Exploration (IDOE) of the National Science Foundation (NSF Grant GX41960). I thank particularly C. P. Summerhayes for his help in reduction of some of the seismic profiles and the 3.5-kHz records, Elazar Uchupi, Roberto Fainstein, Robert Leyden, and John Ewing for their critical comments on the manuscript. Seismic data collected on the REMAC cruises would not have been possible without help of many technicians and watchstanders; in particular, I thank Donald Koelsch, Norival Ferrari, Roberto Fainstein, and Earl Young for design and maintenance of the sparker system.

Copyright 1997 American Association of Petroleum Geologists

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